1. Field of the Invention
The present invention relates generally to a microwaveable adhesive material charge, which is usefully heated by means of microwave radiation, to provide a hot melt adhesive material for dispensing.
2. Description of the Related Art
In the use of hot melt adhesives and other flowable materials which are desirably dispensed in heated condition at the locus of application, the packaging of the heatable, flowable (at least in the heated state) material is an aspect of fundamental importance in the deployment of the material.
Many of such materials immediately prior to their dispensing at the application locus are suitably heated by microwave, ultrasonic, infrared, or other thermal radiation means, to provide the material at the appropriate temperature for the dispensing operation, e.g., in a readily flowable or self-leveling state. For such heating, it is preferable to deploy the material in a heatable form, in a package in which it may also be heated as well as subsequently dispensed.
In the provision of such packaging for sequential storage, heating and dispensing of heated materials, a wide variety of packaging designs has been proposed by the prior art for microwave-heated materials, such as hot melt adhesives which are provided in solid stick form and which under microwave irradiation melt to form a flowable hot melt adhesive medium for bonding and sealant applications.
International Patent Application No. PCT/US91/08661 published 11 Jun. 1992 for "Microwave Hot Melt Adhesive Package and Dispenser," describes a package including a flexible pouch defining a chamber containing the hot melt adhesive, with a dispensing means provided as part of the pouch to permit squeezing dispensing of the hot melt adhesive. The package may include an insulating jacket for facilitating the handling of the package, particularly when the adhesive is in the elevated temperature melt form. The insulating jacket may be formed of polypropylene foam or preferably a foam blend of polystyrene and polyphenylene oxide which may be adhered to and laminated with the microwave transparent layer of the container. In the embodiment shown in FIGS. 1-3 of the patent, the insulating jacket comprises an insulating layer which is heat sealed with a microwave transparent layer at edge portions thereof. The hot melt adhesive employed in such package and dispenser is described to be of varying type (e.g., a water-retaining type which does not require microwave susceptors, or alternatively a type including microwave susceptors in the form of microwave susceptor particles blended or mixed in the hot melt adhesive medium).
International Patent Application No. PCT/US92/05604 published 21 Jan. 1993 discloses a microwave activatable adhesive article including the hot-melt or heat-curable adhesive, and a microwave susceptor layer of at least electrically semi-conductive microwave radiation absorbing material. The susceptor layer is disposed on at least a portion of the substrate, and is responsive to exposure to microwave radiation for raising the temperature of the substrate above a desired level sufficient to melt the substrate.
U.S. Pat. No. 5,188,256 issued 23 Feb. 1993 to J. R. Nottingham, et al. discloses a hot melt adhesive dispenser including a container having hot melt adhesive therein, and a susceptor comprising metal particles adhered to a film such as a high temperature polyimide film, wherein the susceptor is in heat transfer relationship with the hot melt adhesive. The hot melt adhesive container may be formed of a flexible film material having high temperature resistance, and the susceptor may be provided on an interior surface of the container, or otherwise in heat transfer relationship to the hot melt adhesive material. The dispenser disclosed in this patent may further comprise a cover of relatively rigid, heat insulating composite material, such as a foam polystyrene laminated with a bleached hardwood craft paper.
Other hot melt adhesive dispensers are disclosed in co-pending U.S. patent application Ser. Nos. 08/020,511 now abandoned; 08/020,622 now abandoned; and 08/200,852 now U.S. Pat. No. 5,368,199, which variously disclose dispensers in which microwave susceptors are a component of the container for the hot melt adhesive. U.S. application Ser. No. 08/200,852 discloses the concept of a reusable package or sleeve accommodating receipt therein of a hot melt adhesive container including as a component of the container a microwave susceptor material.
In the use of microwaveable hot-melt adhesive materials, the adhesive medium is typically packaged in solid form, comprising a solid, e.g., tubular-shaped, body of the adhesive solid, in a container of suitable material, such as injection-molded plastic, thin film material, or other packaging material, and the resulting adhesive article is termed a "glue stick."
Glue stick articles in prior art practice have incorporated therein a susceptor (sometimes termed "receptor," the terms "susceptor" and "receptor" being used synonymously in the art to denote a radiation-sorptive, e.g., microwave-sorptive, material which on radiation exposure converts radiation energy to heat), typically as a dispersed material or other component in the adhesive medium, or else as a part or component of the packaging material or other non-adhesive structure of the glue stick.
The glue stick may for example comprise a silicone tube having a microwave receptor dispersed in the silicone material as the container. Such tube may have a solid (hot melt) adhesive body of corresponding suitable size and shape disposed in the tube, with a silicone plug being placed in the open end of the tube, after insertion of the hot melt adhesive stick into the tube, for sealing thereof.
A problem which has arisen in the use of glue sticks of hot melt adhesive is the problem attendant the heating of air present in the glue stick, during the microwave heating of the glue stick. When the glue stick is heated, air, typically trapped at the bottom of the container holding the initially solid adhesive medium, also becomes heated, concurrently with the heating of the adhesive medium.
Such air invariably is present in the container holding the initially solid adhesive medium, being unavoidably incorporated in the container upon loading of the solid adhesive body in the container during the manufacture of the adhesive charge.
As it becomes heated during the microwave heating operation, the trapped air expands, erupting the molten glue, and thereby presenting a disadvantage in respect of the evulsion of molten glue on the surrounding environs, such as the microwave oven in which the glue stick charge is disposed in the receptor.
Further, in glue stick articles such as the silicone tube/adhesive solid body/silicone plug construction illustratively described above, there may be a relatively higher concentration of susceptor (receptor) material at the lower portion of the tube as a result of its fabrication in the described manner. In consequence, heating takes place preferentially in the lower portion of the glue stick during microwaving of the adhesive charge, so that the air trapped in the bottom of the glue stick article is even more highly heated by the concentrated susceptor/receptor material, before substantial heating of the hot melt in the upper regions of the adhesive charge takes place, exacerbating the hot melt adhesive eruption problem discussed above.
It therefore is an object of the present invention to provide an improved microwaveable adhesive charge comprising a container or package in which is disposed a microwaveable adhesive medium.
It is another object of the invention to provide an improved adhesive charge in the form of a glue stick article which overcomes the aforementioned molten glue eruption problem.
Other objects and advantages of the present invention will be more fully apparent from the ensuing disclosure and appended claims.